Frontend Development Patterns

1// ✅ GOOD: Component composition
2interface CardProps {
3  children: React.ReactNode
4  variant?: 'default' | 'outlined'
5}
6
7export function Card({ children, variant = 'default' }: CardProps) {
8  return <div className={`card card-${variant}`}>{children}</div>
9}
10
11export function CardHeader({ children }: { children: React.ReactNode }) {
12  return <div className="card-header">{children}</div>
13}
14
15export function CardBody({ children }: { children: React.ReactNode }) {
16  return <div className="card-body">{children}</div>
17}
18
19// Usage
20<Card>
21  <CardHeader>Title</CardHeader>
22  <CardBody>Content</CardBody>
23</Card>

1interface TabsContextValue {
2  activeTab: string
3  setActiveTab: (tab: string) => void
4}
5
6const TabsContext = createContext<TabsContextValue | undefined>(undefined)
7
8export function Tabs({ children, defaultTab }: {
9  children: React.ReactNode
10  defaultTab: string
11}) {
12  const [activeTab, setActiveTab] = useState(defaultTab)
13
14  return (
15    <TabsContext.Provider value={{ activeTab, setActiveTab }}>
16      {children}
17    </TabsContext.Provider>
18  )
19}
20
21export function TabList({ children }: { children: React.ReactNode }) {
22  return <div className="tab-list">{children}</div>
23}
24
25export function Tab({ id, children }: { id: string, children: React.ReactNode }) {
26  const context = useContext(TabsContext)
27  if (!context) throw new Error('Tab must be used within Tabs')
28
29  return (
30    <button
31      className={context.activeTab === id ? 'active' : ''}
32      onClick={() => context.setActiveTab(id)}
33    >
34      {children}
35    </button>
36  )
37}
38
39// Usage
40<Tabs defaultTab="overview">
41  <TabList>
42    <Tab id="overview">Overview</Tab>
43    <Tab id="details">Details</Tab>
44  </TabList>
45</Tabs>

1interface DataLoaderProps<T> {
2  url: string
3  children: (data: T | null, loading: boolean, error: Error | null) => React.ReactNode
4}
5
6export function DataLoader<T>({ url, children }: DataLoaderProps<T>) {
7  const [data, setData] = useState<T | null>(null)
8  const [loading, setLoading] = useState(true)
9  const [error, setError] = useState<Error | null>(null)
10
11  useEffect(() => {
12    fetch(url)
13      .then(res => res.json())
14      .then(setData)
15      .catch(setError)
16      .finally(() => setLoading(false))
17  }, [url])
18
19  return <>{children(data, loading, error)}</>
20}
21
22// Usage
23<DataLoader<Market[]> url="/api/markets">
24  {(markets, loading, error) => {
25    if (loading) return <Spinner />
26    if (error) return <Error error={error} />
27    return <MarketList markets={markets!} />
28  }}
29</DataLoader>

1export function useToggle(initialValue = false): [boolean, () => void] {
2  const [value, setValue] = useState(initialValue)
3
4  const toggle = useCallback(() => {
5    setValue(v => !v)
6  }, [])
7
8  return [value, toggle]
9}
10
11// Usage
12const [isOpen, toggleOpen] = useToggle()

1interface UseQueryOptions<T> {
2  onSuccess?: (data: T) => void
3  onError?: (error: Error) => void
4  enabled?: boolean
5}
6
7export function useQuery<T>(
8  key: string,
9  fetcher: () => Promise<T>,
10  options?: UseQueryOptions<T>
11) {
12  const [data, setData] = useState<T | null>(null)
13  const [error, setError] = useState<Error | null>(null)
14  const [loading, setLoading] = useState(false)
15
16  const refetch = useCallback(async () => {
17    setLoading(true)
18    setError(null)
19
20    try {
21      const result = await fetcher()
22      setData(result)
23      options?.onSuccess?.(result)
24    } catch (err) {
25      const error = err as Error
26      setError(error)
27      options?.onError?.(error)
28    } finally {
29      setLoading(false)
30    }
31  }, [fetcher, options])
32
33  useEffect(() => {
34    if (options?.enabled !== false) {
35      refetch()
36    }
37  }, [key, refetch, options?.enabled])
38
39  return { data, error, loading, refetch }
40}
41
42// Usage
43const { data: markets, loading, error, refetch } = useQuery(
44  'markets',
45  () => fetch('/api/markets').then(r => r.json()),
46  {
47    onSuccess: data => console.log('Fetched', data.length, 'markets'),
48    onError: err => console.error('Failed:', err)
49  }
50)

1export function useDebounce<T>(value: T, delay: number): T {
2  const [debouncedValue, setDebouncedValue] = useState<T>(value)
3
4  useEffect(() => {
5    const handler = setTimeout(() => {
6      setDebouncedValue(value)
7    }, delay)
8
9    return () => clearTimeout(handler)
10  }, [value, delay])
11
12  return debouncedValue
13}
14
15// Usage
16const [searchQuery, setSearchQuery] = useState('')
17const debouncedQuery = useDebounce(searchQuery, 500)
18
19useEffect(() => {
20  if (debouncedQuery) {
21    performSearch(debouncedQuery)
22  }
23}, [debouncedQuery])

1interface State {
2  markets: Market[]
3  selectedMarket: Market | null
4  loading: boolean
5}
6
7type Action =
8  | { type: 'SET_MARKETS'; payload: Market[] }
9  | { type: 'SELECT_MARKET'; payload: Market }
10  | { type: 'SET_LOADING'; payload: boolean }
11
12function reducer(state: State, action: Action): State {
13  switch (action.type) {
14    case 'SET_MARKETS':
15      return { ...state, markets: action.payload }
16    case 'SELECT_MARKET':
17      return { ...state, selectedMarket: action.payload }
18    case 'SET_LOADING':
19      return { ...state, loading: action.payload }
20    default:
21      return state
22  }
23}
24
25const MarketContext = createContext<{
26  state: State
27  dispatch: Dispatch<Action>
28} | undefined>(undefined)
29
30export function MarketProvider({ children }: { children: React.ReactNode }) {
31  const [state, dispatch] = useReducer(reducer, {
32    markets: [],
33    selectedMarket: null,
34    loading: false
35  })
36
37  return (
38    <MarketContext.Provider value={{ state, dispatch }}>
39      {children}
40    </MarketContext.Provider>
41  )
42}
43
44export function useMarkets() {
45  const context = useContext(MarketContext)
46  if (!context) throw new Error('useMarkets must be used within MarketProvider')
47  return context
48}

1// ✅ useMemo for expensive computations
2const sortedMarkets = useMemo(() => {
3  return markets.sort((a, b) => b.volume - a.volume)
4}, [markets])
5
6// ✅ useCallback for functions passed to children
7const handleSearch = useCallback((query: string) => {
8  setSearchQuery(query)
9}, [])
10
11// ✅ React.memo for pure components
12export const MarketCard = React.memo<MarketCardProps>(({ market }) => {
13  return (
14    <div className="market-card">
15      <h3>{market.name}</h3>
16      <p>{market.description}</p>
17    </div>
18  )
19})

1import { lazy, Suspense } from 'react'
2
3// ✅ Lazy load heavy components
4const HeavyChart = lazy(() => import('./HeavyChart'))
5const ThreeJsBackground = lazy(() => import('./ThreeJsBackground'))
6
7export function Dashboard() {
8  return (
9    <div>
10      <Suspense fallback={<ChartSkeleton />}>
11        <HeavyChart data={data} />
12      </Suspense>
13
14      <Suspense fallback={null}>
15        <ThreeJsBackground />
16      </Suspense>
17    </div>
18  )
19}

1import { useVirtualizer } from '@tanstack/react-virtual'
2
3export function VirtualMarketList({ markets }: { markets: Market[] }) {
4  const parentRef = useRef<HTMLDivElement>(null)
5
6  const virtualizer = useVirtualizer({
7    count: markets.length,
8    getScrollElement: () => parentRef.current,
9    estimateSize: () => 100,  // Estimated row height
10    overscan: 5  // Extra items to render
11  })
12
13  return (
14    <div ref={parentRef} style={{ height: '600px', overflow: 'auto' }}>
15      <div
16        style={{
17          height: `${virtualizer.getTotalSize()}px`,
18          position: 'relative'
19        }}
20      >
21        {virtualizer.getVirtualItems().map(virtualRow => (
22          <div
23            key={virtualRow.index}
24            style={{
25              position: 'absolute',
26              top: 0,
27              left: 0,
28              width: '100%',
29              height: `${virtualRow.size}px`,
30              transform: `translateY(${virtualRow.start}px)`
31            }}
32          >
33            <MarketCard market={markets[virtualRow.index]} />
34          </div>
35        ))}
36      </div>
37    </div>
38  )
39}

1interface FormData {
2  name: string
3  description: string
4  endDate: string
5}
6
7interface FormErrors {
8  name?: string
9  description?: string
10  endDate?: string
11}
12
13export function CreateMarketForm() {
14  const [formData, setFormData] = useState<FormData>({
15    name: '',
16    description: '',
17    endDate: ''
18  })
19
20  const [errors, setErrors] = useState<FormErrors>({})
21
22  const validate = (): boolean => {
23    const newErrors: FormErrors = {}
24
25    if (!formData.name.trim()) {
26      newErrors.name = 'Name is required'
27    } else if (formData.name.length > 200) {
28      newErrors.name = 'Name must be under 200 characters'
29    }
30
31    if (!formData.description.trim()) {
32      newErrors.description = 'Description is required'
33    }
34
35    if (!formData.endDate) {
36      newErrors.endDate = 'End date is required'
37    }
38
39    setErrors(newErrors)
40    return Object.keys(newErrors).length === 0
41  }
42
43  const handleSubmit = async (e: React.FormEvent) => {
44    e.preventDefault()
45
46    if (!validate()) return
47
48    try {
49      await createMarket(formData)
50      // Success handling
51    } catch (error) {
52      // Error handling
53    }
54  }
55
56  return (
57    <form onSubmit={handleSubmit}>
58      <input
59        value={formData.name}
60        onChange={e => setFormData(prev => ({ ...prev, name: e.target.value }))}
61        placeholder="Market name"
62      />
63      {errors.name && <span className="error">{errors.name}</span>}
64
65      {/* Other fields */}
66
67      <button type="submit">Create Market</button>
68    </form>
69  )
70}

1interface ErrorBoundaryState {
2  hasError: boolean
3  error: Error | null
4}
5
6export class ErrorBoundary extends React.Component<
7  { children: React.ReactNode },
8  ErrorBoundaryState
9> {
10  state: ErrorBoundaryState = {
11    hasError: false,
12    error: null
13  }
14
15  static getDerivedStateFromError(error: Error): ErrorBoundaryState {
16    return { hasError: true, error }
17  }
18
19  componentDidCatch(error: Error, errorInfo: React.ErrorInfo) {
20    console.error('Error boundary caught:', error, errorInfo)
21  }
22
23  render() {
24    if (this.state.hasError) {
25      return (
26        <div className="error-fallback">
27          <h2>Something went wrong</h2>
28          <p>{this.state.error?.message}</p>
29          <button onClick={() => this.setState({ hasError: false })}>
30            Try again
31          </button>
32        </div>
33      )
34    }
35
36    return this.props.children
37  }
38}
39
40// Usage
41<ErrorBoundary>
42  <App />
43</ErrorBoundary>

1import { motion, AnimatePresence } from 'framer-motion'
2
3// ✅ List animations
4export function AnimatedMarketList({ markets }: { markets: Market[] }) {
5  return (
6    <AnimatePresence>
7      {markets.map(market => (
8        <motion.div
9          key={market.id}
10          initial={{ opacity: 0, y: 20 }}
11          animate={{ opacity: 1, y: 0 }}
12          exit={{ opacity: 0, y: -20 }}
13          transition={{ duration: 0.3 }}
14        >
15          <MarketCard market={market} />
16        </motion.div>
17      ))}
18    </AnimatePresence>
19  )
20}
21
22// ✅ Modal animations
23export function Modal({ isOpen, onClose, children }: ModalProps) {
24  return (
25    <AnimatePresence>
26      {isOpen && (
27        <>
28          <motion.div
29            className="modal-overlay"
30            initial={{ opacity: 0 }}
31            animate={{ opacity: 1 }}
32            exit={{ opacity: 0 }}
33            onClick={onClose}
34          />
35          <motion.div
36            className="modal-content"
37            initial={{ opacity: 0, scale: 0.9, y: 20 }}
38            animate={{ opacity: 1, scale: 1, y: 0 }}
39            exit={{ opacity: 0, scale: 0.9, y: 20 }}
40          >
41            {children}
42          </motion.div>
43        </>
44      )}
45    </AnimatePresence>
46  )
47}

1export function Dropdown({ options, onSelect }: DropdownProps) {
2  const [isOpen, setIsOpen] = useState(false)
3  const [activeIndex, setActiveIndex] = useState(0)
4
5  const handleKeyDown = (e: React.KeyboardEvent) => {
6    switch (e.key) {
7      case 'ArrowDown':
8        e.preventDefault()
9        setActiveIndex(i => Math.min(i + 1, options.length - 1))
10        break
11      case 'ArrowUp':
12        e.preventDefault()
13        setActiveIndex(i => Math.max(i - 1, 0))
14        break
15      case 'Enter':
16        e.preventDefault()
17        onSelect(options[activeIndex])
18        setIsOpen(false)
19        break
20      case 'Escape':
21        setIsOpen(false)
22        break
23    }
24  }
25
26  return (
27    <div
28      role="combobox"
29      aria-expanded={isOpen}
30      aria-haspopup="listbox"
31      onKeyDown={handleKeyDown}
32    >
33      {/* Dropdown implementation */}
34    </div>
35  )
36}

1export function Modal({ isOpen, onClose, children }: ModalProps) {
2  const modalRef = useRef<HTMLDivElement>(null)
3  const previousFocusRef = useRef<HTMLElement | null>(null)
4
5  useEffect(() => {
6    if (isOpen) {
7      // Save currently focused element
8      previousFocusRef.current = document.activeElement as HTMLElement
9
10      // Focus modal
11      modalRef.current?.focus()
12    } else {
13      // Restore focus when closing
14      previousFocusRef.current?.focus()
15    }
16  }, [isOpen])
17
18  return isOpen ? (
19    <div
20      ref={modalRef}
21      role="dialog"
22      aria-modal="true"
23      tabIndex={-1}
24      onKeyDown={e => e.key === 'Escape' && onClose()}
25    >
26      {children}
27    </div>
28  ) : null
29}